As is well known, necessary optical signals are effectively transmitted through a wavelength division multiplexing system in the optical fiber transmission system for the optical communication in which a plurality of lights having different wavelength regions are simultaneously transmitted via one optical fiber cable through an optical wavelength multiplexer that is a filter type bandpass filter using an interference filter utilizing effectively lights, and the lights are divided through an optical wavelength demultiplexer having the same structure as the optical multiplexer into the light of each wavelength region. However, it is necessary that multiplexing and demultiplexing should be carried out upon preparing a bandpass filter corresponding to the plurality of wavelength regions in order to ensure excellent transmission.
For example, as shown in FIG. 43, an optical wavelength demultiplexer 101 demultiplexing two wavelengths λ2 and λ3 out of the light including four wavelengths λ1, λ2, λ3 and λ4 has a structure in which four pieces of optical fibers 102 are provided on both opposite sides of a glass block 103 via a spacer 104 and a lens 105 respectively, and a first bandpass filter 106 and a second bandpass filter 107 corresponding to the wavelengths λ2 and λ3 to be demultiplexed respectively are provided between the spacer 104 on the predetermined position and the glass block 103. In the optical wavelength demultiplexer 101, the light of wavelength λ2 is demultiplexed by the first bandpass filter portion 106 and the light of wavelength λ3 is demultiplexed by the second bandpass filter portion 107 out of the light including four wavelengths λ1, λ2, λ3, and λ4 sent through one optical fiber 102, as the optical axis is designated by the dashed line and the direction of light transmission is designated by the arrow in the figure. The optical wavelength multiplexer has the same structure as the optical wavelength demultiplexer, but the direction of light transmission thereof is reverse to that of the demultiplexer.
Therefore, in the case that lights having a plurality of different wavelength regions are simultaneously transmitted via one optical fiber cable, it is required that bandpass filters corresponding to each wavelength region need not be prepared and moreover excellent transmission of light signals can be carried out. To the requirement mentioned above, a filter that transmits a plurality of lights having different wavelength regions, for example, a filter having two transmission bands and three rejection bands, which can transmit lights having two different wavelength regions, has been proposed (For example, refer to Japanese Patent Publication Hei5-26162). For the filter that can multiplex or demultiplex a plurality of lights having different wavelength regions, it is necessary that isolation between respective different wavelength regions should be sufficiently large and highly reliable because crosstalk would be generated and then the quality of communication would be deteriorated if isolation thereof is not sufficient.
On the other hand, for image pickup of color images for color TV etc., an object image is separated into blue, green and red lights. Then, blue, green and red image signals are obtained by devices provided corresponding to each color. A color separation prism and trimming filters for each color to separate the object image into blue, green and red colors are utilized. The trimming filter is used for perfect color separation because a dichroic film formed on the color separation prism cannot carry out color separation sufficiently by itself. For the trimming filter like this, a filter having a plurality of transmitting regions that can transmit e.g. blue, green and red wavelength regions but rejects the rest of wavelength regions has been developed (e.g. Japanese Patent Publication Shou60-38682 and Japanese Patent Publication Shou60-38683).
It is necessary that isolation between respective transmitting bands should be large enough even though one filter can transmit blue, green and red wavelength regions. Furthermore, it is required that even one filter can carry out color separation so that color separation and color reproduction with high fidelity can be accomplished.
The present invention is put into practice in the light of the circumstance mentioned above and intended to provide a highly reliable multi-bandpass filter having sufficient isolation between a plurality of transmission bands, which can separate surely a plurality of lights with different wavelength regions through one filter and carry out multiplexing and demultiplexing without decreasing the quality of communication in e.g. the wavelength multiplexing system of the optical communication. It is also intended to provide a filter concerning three primary colors in the image and color fields.